AU2012351635B2

AU2012351635B2 - A method of forming a gypsum based product

Info

Publication number: AU2012351635B2
Application number: AU2012351635A
Authority: AU
Inventors: Jean-Luc Germain; Jean-Louis Mongrolle
Original assignee: Saint Gobain Placo SAS
Current assignee: Saint Gobain Placo SAS
Priority date: 2011-12-15
Filing date: 2012-12-13
Publication date: 2016-02-25
Anticipated expiration: 2032-12-13
Also published as: KR101953649B1; EP3599227A1; JP6158211B2; EP2791074A1; GB201121589D0; CA2860907A1; GB2497574A; KR20140114368A; SG11201404052YA; CN104144895A; HK1199241A1; BR112014014564A2; IN2014KN01463A; JP2015504837A; BR112014014564B1; US9242870B2; CA2860907C; PL3599227T3; AU2012351635A1; ES2768087T3

Abstract

A method of forming a gypsum based product is disclosed. The method comprises the steps of: calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with additional water to produce a settable slurry, which is arranged to set to form the gypsum based product.

Description

1 A Method Of Forming A Gypsum Based Product The present disclosure relates to a method of forming a gypsum based product. 5 Definition In the specification the term "comprising" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the term 10 "comprising" such as "comprise" and "comprises". Gypsum occurs naturally as a raw material in the form of calcium sulphate dihydrate. Gypsum containing products, such as plasterboard, are prepared by forming a mixture of calcined or dehydrated gypsum, namely calcium sulphate hemihydrate, with water, to form 15 a settable slurry which is then cast into a pre-determined shape. The hemihydrate reacts with the water and becomes re-hydrated to the dihydrate crystal, which is then cured or dried to the solid state. The hemihydrate form of gypsum is known to depend on the calcination process, and is 20 categorised into two basic forms, the alpha-hemihydrate and the beta-hemihydrate. The beta-hemihydrate is typically formed by heating gypsum under atmospheric conditions, to drive off any moisture and chemically combined water to form dried crystals, which may then be ground to a fine powder. The beta-hemihydrate has long been the favoured hemihydrate in the production of gypsum wall boards or plasterboards, owing to the rapid 25 re-hydration of the ground crystal in forming the slurry. However, the gypsum product derived from the beta-hemihydrate is typically soft and the beta-hydrate requires large volumes of water to produce the required slurry fluidity. The alpha-hemihydrate is formed by heating gypsum under pressure to similarly remove 30 the water associated therewith. The gypsum product derived from the alpha-hemihydrate however, is found to be harder and comprises a higher strength and density compared with beta-hem ihydrate derived gypsum.

2 It is necessary to use substantial amounts of water in gypsum slurries in order to ensure proper flowability of the slurry. Unfortunately, most of this water must eventually be driven off by heating, which is expensive due to the high cost of the fuels used in the heating process. The heating step is also time-consuming. It is known that alpha-hemihydrate 5 has a substantially lower water demand than beta-hemihydrate, meaning that if alpha hemihydrate could be used in making wallboard, it would substantially reduce the water demand and hence the expense and time required to produce the wallboard. This is a further advantage associated with alpha-hem ihydrate. 10 However, alpha-hemihydrate is not generally used commercially in the production of gypsum wallboard primarily due to its slower hydration rate compared to beta hemihydrate, which would therefore require a slower passage of boards along the production line. 15 W02007/084346 discloses a method for the production of alpha-type gypsum. A gypsum slurry is delivered into an autoclave, where it is heated typically to a temperature of about 280OF (about 1370C) under a pressure of 3-4 Bar and converted to alpha hemihydrate. The slurry exits the autoclave via a pressure let down valve and is delivered to a flash tank, where it is cooled and excess steam is collected. 20 US2008/0069762 discloses a process for making a blend of alpha- and beta-stucco. The process includes a slurry calcination step in which gypsum slurry is held in a reactor at a temperature of e.g. 1490C and a pressure of e.g. 3.4 to 4.8 Bar. The partially calcined gypsum product discharges from the reactor as a slurry comprising calcium sulphate 25 dihydrate and alpha calcium sulphate hemihydrate and feeds an accumulator tank, which acts as a holding tank and permits release of the steam as the slurry's pressure drops to atmospheric pressure. The slurry then discharges from the accumulator tank and feeds a de-watering unit which removes water to produce a dewatered solids-containing product and a removed water stream. The dewatered product has a 2-6wt% free water moisture 30 content. The dewatered product is fed to a board stucco kettle calciner at conditions to convert the majority or all of the gypsum in the dewatered product to beta calcium sulphate hemihydrate.

3 The reference to prior art in the background above is not and should not be taken as an acknowledgment or any form of suggestion that the referenced prior art forms part of the common general knowledge in Australia or in any other country. 5 In accordance with the present disclosure as seen from a first aspect, there is provided a method of forming a gypsum based product, the method comprising the steps of: calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with 10 additional water to produce a settable slurry, which is arranged to set to form the gypsum based product. In a particular aspect, there is disclosed a method of forming a gypsum based product, the method comprising the steps of: 15 calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; reducing the water content of the alpha-hemihydrate slurry to provide a separate water stream from the alpha-hemihydrate slurry; and subsequently passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with 20 additional water to produce a settable slurry, which is arranged to set to form the gypsum based product; wherein the step of passing the alpha-hemihydrate slurry from the vessel to the mixer comprises the step of maintaining the temperature of the alpha-hemihydrate slurry at over 700C. 25 Advantageously, the method eliminates the requirement for drying of the hemihydrate, which is associated with the conventional formation of the settable slurry, and therefore reduces the energy demands of the method. Moreover, the reduced water amounts associated with achieving the desired fluidity of alpha-hemihydrate settable slurry 30 compared with beta-hemihydrate, provides a further energy saving since less water must be removed during the drying of the gypsum product.

4 Preferably, the calcination step comprises substantially filling the vessel with water and gypsum so that the vessel is substantially devoid of free space, such that the water produced during the calcination of gypsum is prevented from evaporating. 5 Preferably, the raised temperature comprises a temperature within the range between 1100C and 1700C, preferably between 1200C and 1500C, more preferably between 1300C and 1400C. Typically, the pressure is adjusted in accordance with the operating temperature, such that 10 the operating pressure corresponds to the vapour pressure of steam at the operating temperature. Preferably, the raised pressure comprises a pressure within the range 2-8 Bars, more preferably 3-5 Bars. The method preferably further comprises the step of cooling the alpha-hemihydrate slurry 15 after the calcination step. Typically, the step of cooling the alpha-hemihydrate slurry takes place while the alpha-hemihydrate slurry is still held at the raised pressure of about 2 to 8 Bars. Typically, the step of cooling the alpha-hemihydrate slurry is carried out using a heat exchanger. Preferably, the alpha-hemihydrate is cooled to a temperature less than 100C, such as 900C. 20 Preferably, after the step of cooling the alpha-hemihydrate slurry, the slurry is de pressured to a pressure of 1 Bar (i.e. atmospheric pressure). Typically, the method further comprises the step of substantially separating the water from 25 the alpha-hemihydrate slurry after the step of reducing the pressure acting on the alpha hemihydrate slurry. This may be done e.g. using a belt filter or a centrifugal separator, i.e. hydroclone. Preferably, in this case, the separated water is circulated for mixture with a fresh quantity of gypsum, for introducing into the vessel to start a further calcination process. In this case, the heat energy contained within the separated water reduces the 30 requirement for heating of the vessel to achieve the raised temperature of e.g. 110 C to 1800C.

5 In the case that water is separated from the alpha-hemihydrate slurry, the free water content of the remaining slurry is typically 1-30wt%, preferably 5-30wt%, more preferably 8-30wt%. 5 Preferably, the method further comprises the step of grinding the alpha-hemihydrate slurry to reduce the size of particulates therein. It is thought that grinding of the alpha hemihydrate slurry results in a greater reactivity of the alpha-hemihydrate particles, so as to increase the hydration rate of the alpha-hemihydrate particles during the step of forming a settable slurry. The step of grinding the alpha-hemihydrate slurry may be carried out 10 using wet grinding techniques. In this case, the grinding may be carried out at a temperature of 500C or more, preferably 700C or more, more preferably 800C or more. Methods for controlling the size of the alpha-hemihydrate particles during the calcination stage are known in the art and may be used as an alternative or in addition to the grinding 15 step. Preferably, during the step of passing the alpha-hemihydrate slurry from the calcination vessel to the mixer, the temperature of the alpha-hemihydrate slurry is maintained at 700C or more. By maintaining the temperature of the alpha-hemihydrate slurry at this level, it is thought that hydration of the alpha-hemihydrate particles to form a settable gypsum 20 product may be avoided until entry of the alpha-hemihydrate slurry into the mixer. Preferably, the temperature of the alpha-hemihydrate slurry is maintained at over 800C, more preferably over 850C. In addition, it is desirable that the step of passing the alpha-hemihydrate slurry from the 25 calcination vessel to the mixer should not take too long, so as to further avoid hydration of the alpha-hemihydrate particles until entry of the particles into the mixer. Typically the time taken for the alpha-hemihydrate slurry to pass from the calcination vessel to the mixer is less than 120 minutes, preferably less than 60 minutes, more preferably less than 30 minutes. 30 It is thought that the addition of cold water (e.g. about 20C-300C) in the mixer, for mixing with the alpha-hemihydrate slurry will rapidly decrease the temperature of the alpha- 6 hemihydrate slurry, so as to promote the hydration of the alpha-hemihydrate particles to form a settable gypsum product. The method further comprises the addition of one or more further additives to the 5 hemihydrate slurry within the mixer, such as accelerators and foaming agents. Preferably, the gypsum based product comprises a gypsum board. In accordance with the present disclosure as seen from a second aspect, there is provided 10 a method of forming a gypsum based product, the method comprising the steps of: calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with additional water to produce a settable slurry, which is arranged to set to form the gypsum 15 based product, wherein the alpha-hemihydrate slurry is passed from the vessel to the mixer without undergoing a drying stage. Preferred features of the method of the second aspect may comprise one or more of the preferred features of the method of the first aspect. 20 An embodiment of the present disclosure will now be described by way of example only and with reference to the accompanying drawing which provides a schematic illustration of the steps associated with the method according to an embodiment of the present disclosure. 25 Referring to the drawing, the method according to an embodiment of the present disclosure comprises the initial step of forming a mixture 20 of water and gypsum 10 in a ratio of approximately 1 part gypsum to 1.5 parts water, pressurising the mixture in a pressuriser 30 and pre-heating it by means of a heat exchanger 40 (e.g. a water/water heat exchanger). The mixture is then introduced into a calcination vessel 50, e.g. by 30 means of a pump or in a long water column. The mixture is subsequently heated to a temperature in the range 130OC-1400C and the vessel 50 is pressurised to a pressure in the range 3-5 Bars. The vessel 50 is substantially filled with the mixture to remove any free space therein, such that the water within the vessel 50 and principally the water derived 7 from the calcining of the gypsum, is prevented from evaporating and thus escaping from the vessel 50. Following the calcination stage the resulting mixture of water and alpha-hemihydrate slurry 5 60 is cooled to a temperature of approximately 900C using the heat exchanger 40, de pressurised in a de-pressuriser 70 and passed to a separation unit 80, wherein the hemihydrate slurry is substantially separated from the water. The water is circulated from the separation unit 80 back to the vessel 50 to preheat subsequent water and gypsum before entering the vessel 50 and thus reduce the energy demands associated with the 10 heating of the mixture. The alpha-hemihydrate slurry comprising approximately 6% water is passed from the separation unit 80 to a mixer 90, for subsequent post processing of the slurry which includes the addition of water 100 and optional additives, such as accelerators 110 (for reducing the setting time) and foaming agents 120 to produce a settable slurry. It is also envisaged however, that the post processing may further comprise the grinding of 15 the hemihydrate slurry (e.g. in a screw grinder 140) to reduce the size of particulates disposed therein before the alpha-hemihydrate slurry is introduced into the mixer 90. In this respect, the hemihydrate slurry is passed to the mixer 90 to achieve the required fluidity and setting characteristics, without undergoing any drying stage, thereby reducing the energy demand in producing the gypsum product. Moreover, the resulting settable 20 slurry comprising 30-40% water which is then passed to a production line 130 for subsequent preparation of a gypsum product such as a plasterboard, will require less curing owing to reduced amounts of water which are required to attain the desired fluidity of the alpha-hemihydrate settable slurry, compared with the amount of water required to attain the desired fluidity of a beta-hemihydrate settable slurry. Since alpha-hemihydrate 25 typically has a slower hydration rate than beta-hemihydrate, the setting times for alpha hemihydrate slurry are typically longer than for beta-hemihydrate slurry. Thus, in the manufacture of gypsum boards, it is generally desirable when using alpha-hemihydrate slurry, to have a longer forming belt to provide sufficient time for setting of the slurry.

Claims

1. A method of forming a gypsum based product, the method comprising the steps of: calcining a mixture of water and gypsum under conditions of raised temperature and pressure within a vessel to produce an alpha-hemihydrate slurry therein; reducing the water content of the alpha-hemihydrate slurry to provide a separate water stream from the alpha-hemihydrate slurry; and subsequently passing the alpha-hemihydrate slurry from the vessel to a mixer for mixing with additional water to produce a settable slurry, which is arranged to set to form the gypsum based product; wherein the step of passing the alpha-hemihydrate slurry from the vessel to the mixer comprises the step of maintaining the temperature of the alpha-hemihydrate slurry at over 700C.

2. A method according to claim 1, wherein the calcination step comprises substantially filling the vessel with water and gypsum so that the vessel is substantially devoid of free space, such that the water produced during the calcination of gypsum is prevented from evaporating.

3. A method according to any one of the preceding claims, wherein the raised temperature comprises a temperature within the range 110 C to 1700C.

4. A method according to any one of the preceding claims, wherein the raised pressure comprises a pressure within the range 2 to 8 Bars.

5. A method according to any one of the preceding claims, further comprising the step of cooling the alpha-hemihydrate slurry after the step of calcining the mixture of water and gypsum.

6. A method according to claim 5, wherein the alpha-hemihydrate slurry is cooled to a temperature less than 100C. 9

7. A method according to claim 5 or claim 6, comprising the further step, after the step of cooling the alpha-hemihydrate slurry, of de-pressurising the slurry.

8. A method according to any one of the preceding claims, further comprising circulating the separated water stream to preheat further gypsum before entering the vessel.

9. A method according to any one of the preceding claims, wherein after the step of reducing the water content of the alpha-hemihydrate slurry, the water content of the alpha hemihydrate slurry lies within a range of 1-1Owt%.

10. A method according to any preceding claim, further comprises the step of grinding the alpha-hemihydrate slurry to reduce the size of particulates therein.

11. A method according to any one of the preceding claims, wherein the time taken for the alpha-hemihydrate slurry to pass from the vessel to the mixer is less than 120 minutes.

12. A method according to any one of the preceding claims, further comprising the addition of one or more additives to the hemihydrate slurry within the mixer, such as an accelerator and/or a foaming agent.

13. A method according to any preceding claim, wherein the gypsum based product comprises a gypsum board.